Electronically controlled universal motor

ABSTRACT

An electric motor apparatus and method of controlling the same are provided. The electric motor apparatus includes a stator and a rotor rotationally mounted coaxially with the stator; an intermediate screen having a magnetic material and rotationally mounted between the stator and the rotor to provide magnetic screening between the rotor and the stator; and control circuitry to control power supplied to windings on the rotor and the stator in dependence upon a desired output rotational speed.

This application is a continuation of U.S. patent application Ser. No.14/080,131 filed Nov. 14, 2013, the entire contents of which areincorporated herein by reference in this application.

TECHNICAL FIELD

The technical field relates to electric motors and in particular tomotors in which both the rotor component and the stator component havewinding arrangements.

BACKGROUND

There are several different ways of configuring electric motors each ofwhich has its own associated advantages and disadvantages. Many of theseconfigurations provide very efficient operations. However, this highefficiency is generally restricted to a particular optimum operatingfrequency of the motor and when the motor moves away from this operatingfrequency the efficiency may drop dramatically. In order to address thisproblem, motors of the prior art have been operated in conjunction withgearing systems, such that they can drive axles at different speeds,while still operating close to their own optimum rotation frequency. Theprovision of gears adds cost to the system and can themselves reduceefficiency.

One known type of motor is the universal motor which is schematicallyshown in FIG. 1 and has a rotor component and stator component that bothhave winding arrangements. Such a universal motor can be operated witheither DC or AC power and generally provides a compact and high poweredmotor. To some extent, a universal motor can be viewed as combining theadvantages of both induction motors and permanent magnet motors.However, the difficulties associated with coupling the requiredwaveforms to the rotor winding arrangements have significant drawbacks.This problem is addressed in U.S. Pat. No. 8,450,954.

Although such a universal motor may provide a compact and high poweredmotor, the efficiency of such a motor still reduces dramatically awayfrom an optimum operating frequency and thus, such a motor is generallyoperated in conjunction with some gearing mechanism.

SUMMARY

A first aspect provides an electric motor apparatus comprising:

a stator component and a rotor component rotationally mounted coaxiallywith and within said stator component;

said stator component and said rotor component each comprising windingsfor generating an electromagnetic field;

said electric motor further comprising an intermediate screeningcomponent rotationally mounted between said stator component and saidrotor component and configured to provide at least some magneticscreening between said rotor component and said stator component;

said intermediate screening component comprising at least somemagnetically active sections configured such that changing magneticfields generated by changing electric currents in said windings oneither said rotor component or said stator component generate a force onsaid magnetically active sections causing said intermediate screeningcomponent to rotate; and

control circuitry for independently controlling power supplied to saidwindings on said rotor component and said stator component in dependenceupon a desired output rotational speed.

The present invention recognises that motors where both the rotor andstator have windings which are independently powered are known andprovide a compact motor which provides a high torque. However, it alsorecognises that there are problems associated with running such motorsefficiently at different rotational speeds. It addresses this problem byplacing a rotatable intermediate screening component between the rotorand stator of such a motor where both the rotor and stator have drivenwindings, such that the magnetic field generated from the rotor andstator which previously interacted to provide the force that drove therotor are decoupled from each other via the intermediate screeninglayer. This allows the rotor and stator to in effect act as independentmotors, the rotor speed depending on the power and direction of thesetwo “motors”. Thus, rather than having a single optimum power loadingwhich provides a particular rotational speed, with other speeds havingmuch lower efficiencies, the motor can run relatively efficiently at aplurality of different speeds. These speeds include the speed generatedwhen the rotor is powered alone at or close to its optimum efficiency,the speed when just the stator is powered at or close to its optimumefficiency and other speeds when the rotor and stator are poweredtogether either to generate rotation in the same direction nor inopposite direction at their high efficiency levels, such that theresulting rotational speed of the rotor is a sum of the rotationalspeeds generated by each of the rotor and stator components.

By amending the motor in this way and extending the range of speeds atwhich it operates efficiently the need for a gearing system is reducedor in some cases eliminated, which decreases the cost and improves theefficiency of the system. Indeed, where a variable gear set is avoided,the need for a corresponding transmission is also avoided. Complextechniques in which for example the gear variability is provided by aplanetary gear set connected to a second motor are thus avoided, andsuch a second motor is not required.

Although, in some embodiments said stator component and said rotorcomponent may comprise a different number of windings, in others theycomprise the same number of windings. As the stator is larger than therotor then it can accommodate more windings and thus, in someembodiments it may be efficient to arrange more windings on the stator.In others, however, it may be advantageous if the rotor and stator havethe same number of windings as this makes the design more symmetricaland thus, in some cases easier to design and to manufacture.

In some embodiments, said intermediate screening component comprises aplurality of inserts on at least one of said inner and outer surfaces ofa highly electrically conductive material, said plurality of insertsrunning at an offset angle to said longitudinal axis to form a squirrelcage, said plurality of inserts on a same surface being coupled ateither end to form loops.

In some embodiments the screening component comprises a squirrel cage ofhighly conductive material such that the current is induced in the loopsof the highly conductive material which then generate a magnetic fieldthemselves. In this way, an inductive motor is produced.

In some embodiments, the plurality of inserts is arranged on both theinner and outer surfaces.

Although, the inserts may be on only one of the surfaces of theintermediate screening component, in many embodiments they will be onboth surfaces with an inductive motor being produced between the outersurface of the intermediate screening component and the stator and theinner surface of the intermediate layer and the rotor.

In some embodiments, said plurality of inserts on said inner surface arearranged at different circumferential positions to said plurality ofinserts on said outer surface.

It may be advantageous if the inserts are arranged at differentcircumferential positions on the inner and outer surfaces. This allowsthe intermediate screening component to be made thinner which can resultin a more compact motor.

The arrangement of the windings on the rotor and the stator are designedto be appropriate for interaction with the arrangement of the inserts.Thus, in some embodiments there may be a same number of windings on therotor and the stator and the same number of inserts on the inner andouter surfaces. In such a case these inserts are preferentially arrangedat different circumferential positions such that inserts on the outersurface for example slot within the gaps between the inserts on theinner surface.

In other embodiments, said intermediate screening component comprisessaid plurality of protrusions on both said inner and outer surfaces.

Rather than having induction motors, the intermediate screen may haveprotrusions and be formed of some magnetically active material and themotor reactors are switched reluctance motors.

In some cases, said inner and said outer surface comprise a same numberof said plurality of protrusions, said plurality of protrusions on saidinner surface being offset compared to said plurality of protrusions onsaid outer surface, such that a protrusion on said inner surface is notformed at a same circumferential position as a protrusion on said outersurface.

Having the protrusions offset with respect to each other allows for athinner and yet robust intermediate screening surface that makesefficient use of the material required to manufacture it.

In some embodiments said protrusions are formed by said inner and outersurfaces being undulating surfaces, said undulating surfaces undulatingat a substantially same rate and same direction such that such that athickness of said intermediate screening component is substantiallyconstant around the circumference.

Having the same number of protrusions on the inner and outer surfacesallows an indent on the outer surface to become a protrusion on theinner surface which reduces the amount of material required to form theintermediate screening component and also reduces its thickness allowingfor a more compact motor. For example, where the number of windings onthe rotor and stator are the same then the intermediate screen can beconfigured to have the same number of protrusions on the inner and outersurface.

In some embodiments, said rotor component and said intermediatescreening component are mounted on a same axle.

Although the intermediate screening component may be mounted on bearingson the rotor itself, it may be advantageous to mount it on the same axleas the rotor. An axle is generally machined to a high degree of accuracyand mounting the rotor and the intermediate screen on the same componentreduces the potential for error tolerances and makes the design andmanufacture of the motor simpler and more accurate.

In some embodiments, said control circuitry comprises a stator componentcontrol unit and a rotor component control unit, said rotor componentcontrol unit being mounted on said rotor component.

In order for the electric motor to function efficiently with theintermediate screening component, then the rotor needs to be separatelycontrolled to the stator. Thus, in embodiments of the present inventionthe rotor component has a control unit mounted on the rotor componentwhich allows separate control of the windings on the rotor component tothose on the stator component. This allows the motor to be controlledefficiently as the stator and rotor can be controlled independently ofeach other allowing each to operate at or close to its most efficientpower loading and in either of the two rotation directions. In this waya number of different output speeds can be generated from the motor athigh efficiency without the need for gearing mechanisms. Furthermore, bymounting the rotor control unit on the rotor it can be independentlycontrolled without the need to transmit control signals to the rotoritself, reducing the need for the commutators or numerous slip rings,since only the externally supplied rotor supply voltage needs to becoupled to the rotor component and. This means that the noise andinefficiency associated with a commutator and the manufacturingcomplexity associated with numerous slip rings are each avoided or atleast reduced.

In some embodiments, said stator control unit is configured to controlpower supplied to said windings on said stator component to therebygenerate a rotational force on said intermediate screening component;and

said rotor control unit is configured to control power supplied to saidwindings on said rotor component to thereby generate a rotational forcebetween said intermediate screening component and said rotor component,said rotational force exerted on said rotor component being dependent ona rotational speed of said intermediate screening component due to saidrotational force generated by said stator component and said rotationalforce generated between said intermediate screening component and saidrotor component by power supplied to said windings of said rotorcomponent.

As noted above, independent control of the power supplied to thewindings on the stator and rotor provides a wide range of controlpossibilities for the motor. In particular the rotational force providedto the intermediate screening component can be varied in a number ofdifferent ways while still providing the windings on the respectiverotor and/or stator with or close to their most efficient power loading.

In some embodiments, said rotational forces generated by said statorcontrol unit and said rotor control unit are combined via saidintermediate screening component to generate a cumulative rotationalforce on said rotor component.

As noted previously, the cumulative rotational force on the rotor isprovided by the force generated from the windings of the rotor componentand the intermediate screening component and the force generated by thestator windings transmitted to the rotor via intermediate screeningcomponent, a cumulative rotational force being generated via thescreening component which is transmitted to the rotor component. Thus,by independently controlling the stator and the rotor windings thecumulative force exerted on the rotor component can be controlled acrossa wide range of values.

In some embodiments, the electric motor further comprises a rotationalposition sensor configured to generate rotational position informationindicative of a relative orientation of said intermediate screeningcomponent and said rotor component and said intermediate screeningcomponent and said stator component,

said rotor control unit mounted on said rotor component being coupled toa power supply supplied to said rotor component and configured togenerate a rotor component power supply waveform from said power supplyin dependence upon said rotational position information and a desiredoutput frequency of rotation of said rotor component relative to saidintermediate screening component and to apply said rotor component powersupply waveform to said rotor component; and

said stator control unit being coupled to a power supply supplied tosaid stator component and configured to generate a stator componentpower supply waveform from said power supply in dependence upon saidrotational position information of said intermediate screening componentand said stator component and a desired output speed of rotation and toapply said stator component power supply waveform to said statorcomponent.

In order to provide effective independent control of the rotor and thestator, particularly for switched reluctance motors, a rotationalposition sensor configured to generate rotational position informationindicative of a relative orientation of the intermediate screeningcomponent and the rotor component and the intermediate screeningcomponent and the stator component is required such that the windingsare powered in an appropriate fashion to drive them with the appropriatecurrents to produce the required force on the intermediate screeningcomponent. Where the intermediate screening component is formed ofsquirrel cages of high conductive inserts then the rotational positionsensor is less important as such inductive motors tend to be selfaligning. However, it may be required in some embodiments where thestator and rotor are interacting to ensure they do so in a correctmanner.

A second aspect of the present invention provides a method of operatingan electric motor apparatus, said electric motor apparatus comprising: astator component and a rotor component rotationally mounted coaxiallywith and within said stator component; said stator component and saidrotor component each comprising windings for generating anelectromagnetic field; said electric motor further comprising anintermediate screening component rotationally mounted between saidstator component and said rotor component and configured to provide atleast some magnetic screening between said rotor component and saidstator component; said intermediate screening component comprising atleast some magnetically active sections configured such that changingmagnetic fields generated by changing electric currents in said windingson either said rotor component or said stator component generate a forceon said magnetically active sections causing said intermediate screeningcomponent to rotate; said method comprising

controlling an output rotational speed of said motor by providing powerto either said rotor or said stator or said rotor and said stator,control circuitry for independently controlling power supplied to saidwindings on said rotor component and said stator component in dependenceupon a desired output rotational speed.

A third aspect of the present invention provides an electric motorapparatus comprising:

a stator component and a rotor component rotationally mounted coaxiallywith and within said stator component;

said stator component and said rotor component each comprising windingsmeans for generating an electromagnetic field;

said electric motor further comprising an intermediate screening meansfor providing at least some magnetic screening between said rotorcomponent and said stator component, said intermediate screening meansbeing rotationally mounted between said stator component and said rotorcomponent;

said intermediate screening means comprising at least some magneticallyactive means for responding to changing magnetic fields generated bychanging electric currents in said windings on either said rotorcomponent or said stator component by generating a force such that saidintermediate screening component is caused to rotate; and

control means for independently controlling power supplied to saidwinding means on said rotor component and said stator component independence upon a desired output rotational speed.

The above, and other objects, features and advantages of this inventionwill be apparent from the following detailed description of illustrativeembodiments which is to be read in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a cross section through motor 10 according to anembodiment of the present invention;

FIG. 1 b shows a number of rotational speeds that can be produced by amotor with rotor and/or stator powered to a high power loadingefficiency;

FIGS. 2 and 3 show cross sections through motors according toembodiments of the present invention;

FIG. 4 shows an example of an intermediate screening component for aninduction motor;

FIG. 5 shows an example of an intermediate screening component for aswitched reluctance motor;

FIG. 6 shows a motor and control circuitry according to an embodiment ofthe present invention;

FIG. 7 shows a portion of the rotor and axle according to an embodimentof the present invention; and

FIG. 8 shows a flow diagram illustrating steps in a method according toan embodiment of the present invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 a shows a cross section through motor 10 according to anembodiment of the present invention. Motor 10 comprises a stator 20mounted via bearings 22 on rotating axle 30. There is also anintermediate screen element 40 mounted on axle 30 via bearings 42. Thereis then a rotating component 50 which is fixably mounted to axle 30 androtates with it.

Stator 20 has longitudinal winding elements 24 on protrusions around itsinner circumference, these windings generating a magnetic field whenpowered.

Rotor 50 itself has windings 54 mounted at various positions along itsouter circumference on protrusions extending from the rotor and whenthese are powered these too generate a magnetic field. The powering ofthe windings 54 on rotor 50 is controlled by control circuitry 56 whilethe powering of the stator windings 24 are controlled by controlcircuitry 26.

Between the stator and the rotor there is an intermediate screeningelement 40. This screening element acts to decouple the magnetic fieldgenerated by the stator from that generated by the rotor. Theintermediate screening element 40 is itself mounted on bearings 42 onaxle 30. By mounting it on the same axle as the stator is mounted andindeed as the rotor is mounted the tolerances of the device are easierto control as the axle is generally a highly machined article.

The intermediate screening element 40 has magnetically active areas 44which interact with the magnetic fields generated by both the rotor andthe stator providing a force which causes the intermediate screeningelement 40 to rotate. In many cases the screening element 40 will havemagnetically active areas on both its inside and its outside surface.

In this regard, the magnetically active areas may consist of protrusionswhere the intermediate screening element is formed of a magneticmaterial such as silicon iron or it may consist of highly conductiveloops arranged in a squirrel cage slanted arrangement in which anelectric current is induced in response to a changing magnetic fieldsuch that a rotational force is exerted on the intermediate screeningelement 40.

By placing an intermediate screening element 40 between the stator 20and the rotor 30 elements the rotor 30 and stator 20 are magneticallydecoupled from each other and this allows the stator 20 and rotor 30elements to be independently controlled and act like two independentmotors. Thus, controlling the power supply to the rotor windingsseparately to the power control to the stator windings providessignificantly increased control of the motor unit. In particular, asnoted previously many of such motor arrangements run at very highefficiencies but only at particular power loadings, that is generallywhere the coils are provided with sufficient power to provide asaturated magnetic field within the protrusion on which they aremounted. Allowing individual control of the stator 20 and the rotor 30means that they can both be powered at or close to their most efficientloading independently of each other. This means that there are severalrotational frequencies that can be generated where the motor isoperating at a high efficiency.

FIG. 1 b shows schematically how such a motor may generate differentrotational speeds while powering the respective windings to their mostefficient power loading. In this example, the stator 20 will generate ahypothetical rotational speed of 10 units when powered to its mostefficient power loading while the rotor will generate 6. Thus, by eitherpowering them alone, or together or together but in opposite directionsa number of different speeds can be obtained. Thus, stator alone gives10 units either clockwise or anti-clockwise. Rotor alone gives 6 units,while stator plus rotor gives 16, and stator minus rotor gives 4. Thus,the potential rotational frequencies output by the motor at highefficiency are increased allowing the motor to operate withouttransmission systems in some cases.

FIG. 2 shows a motor 10 according to an embodiment of the presentinvention in cross section. In this embodiment, there are three coils onthe stator 20 and correspondingly three coils on the rotor 30. There isan intermediate rotating core 40 which acts as the intermediatescreening component and will have some magnetically active componentssuch as magnetic protrusions or high conductive loops on it which arenot shown. The windings on the stator 20 and the rotor 30 areindependently controlled by different processing cores 26 and 56respectively.

FIG. 3 shows an example of a motor 10 according to an embodiment of thepresent invention when a motor is a switched reluctance motor, thestator 20 and the rotor 30 have the same number of coils and theintermediate screening component 40 is formed of a magnetic materialhaving protrusions. In this case, if the protrusions are arranged suchthat the indent on one side forms the protrusion on the other side thisallows the screening element to be made of less material than would bethe case were the protrusions not arranged in this way. This allows aless expensive and more compact motor to be built.

In order for such an arrangement to function efficiently it isadvantageous if the number of coils on the rotor and the stator areequal. It should be clear, that in other embodiments there may be adifferent number of protrusions on the inner and outer surfaces of theintermediate screening component 40 and the number of coils on thestator and the rotor may also be different. In this regard it may beadvantageous to have a different number of coils on the stator and rotoras the stator will naturally have a larger circumference and thus it canaccommodate more coils than the rotor.

FIG. 4 shows an intermediate screening element 44 according to anembodiment of the present invention. In this embodiment, there are highconductive inserts 47 arranged at an angle to the longitudinal axis ofthe screening element in a squirrel caged arrangement. There is aconducting element 48 at either end which connects the inserts togetherallowing for loops to be formed.

In this embodiment, there are highly conductive inserts on both theinside and the outside surfaces of the screening element and they arearranged in this particular example at an offset to each other such thatthey can be inserted onto a thinner screening element than would be thecase if they were arranged at a same circumstantial position.

This is shown schematically in cross section in FIG. 4 where the curvedscreening element is shown as a straight element for convenience ofillustration. Inserts 47 on the outer surface are arranged offset to theinserts 47 that are on the inner surface. In order for these inserts toform loops through which current can flow there are two connecting areasthat run along the inner circumference of this ring type screeningelement and the outer circumference respectively. One of the connectionareas 48 connecting the inserts 47 that are on the inner surfacetogether and the other connecting the inserts that are on the outersurface together.

FIG. 5 shows an alternative screening element 40 that is used inswitched reluctance motors. This screening element 40 is formed of amagnetic material such as silicon steel and contains protrusions on boththe inner and the outer surfaces. When within a varying electric fieldthe induced magnetic field in the protrusions generates a force on thescreening element causing it to rotate. This force is caused in responseto a changing magnetic field generated by the windings on one or more ofthe stator or the rotor.

In this embodiment, the protrusions on one side form the indents on theother side allowing the screen 40 to be made from a smaller amount ofmaterial and in a compact form allowing for a compact motor.

The screen 40 is shown in cross section in FIG. 5 such that it canclearly be seen that an indent on one side produces a protrusion on theother.

FIG. 6 shows motor 10 having rotor 30, screen 40 and stator not shownalthough stator coils 24 are shown. A power supply voltage 27 suppliespower to the stator coil via stator control circuitry 26. The statorcoil control circuitry 26 also receives an input from the rotor supplyvoltage source 125 such that it is aware of the voltage currently beingsupplied to the rotor and from position information indicating therelative position of the screen 40 and the rotor 30.

Motor 50 has a rotor control unit 56 which receives power via slip rings140 from rotor supply voltage source 125. It also receives informationabout the position of the screen 40 relative to the rotor and independence upon this controls the powering to the rotor coils. In thisway, the powering of the rotor and stator coils 54, 24 can be controlleddepending on the relative position of the rotor 50 and the screen 40 toenable a desired output rotation of the axle 30 to be achieved.

FIG. 7 shows the rotor 50 in more detail. There are several slips rings225 and 230 for providing the power and the control signals to the rotor50 via the rotating axle 30. There is also control circuitry 56 in theform of a processor core for controlling the voltage sent to the rotorcoils (not shown) via circuitry 220 which receives a voltage from thepower supply. In this case, circuitry 220 is in the form of switchingcircuitry which switches the power on and off to the respective coils inresponse to signals from the processor core 50 which reacts toinformation that it receives regarding the relative position of therotor 50 to the intermediate screening component (not shown).

FIG. 8 shows a flow diagram illustrating steps in the method accordingto an embodiment of the present invention. Initially a desiredrotational speed is selected and it is determined whether it is in theclockwise or anti clockwise direction. Depending on the requireddirection of the rotation, the polarity of the power to be supplied tothe windings is determined

It is then determined if the required rotational speed is a high speed.If it is then the stator and the rotor windings are both powered withcurrent flowing in a direction that will generate the required directionof rotation. If it is a lower speed then one of the stator or the rotorwindings may be powered on their own. In this regard, depending on theconfiguration, powering either the windings on the stator or the rotormay generate a different speed (see FIG. 1 b for example). If the actualspeed required is lower than the speed generated by either the rotor orstator powered alone then it may be appropriate to power one in onedirection and the other in the opposite direction, such that theygenerate opposing rotational forces on the screening element, and theresulting rotation of the axle is the difference between the two forces.

Had the anti-clockwise direction been selected initially then the samesteps would have been performed except for with the current runningthrough the coils in the opposite direction to generate a force in theopposite direction.

Although illustrative embodiments have been described in detail hereinwith reference to the accompanying drawings, it is to be understood thatthe claims are not limited to those precise embodiments, and thatvarious changes and modifications can be effected therein by one skilledin the art without departing from the scope and spirit of the appendedclaims. For example, various combinations of the features of thefollowing dependent claims could be made with the features of theindependent claims.

I claim:
 1. An electric motor apparatus comprising: a stator and a rotorrotationally mounted coaxially with said stator; an intermediate screencomprising a magnetic material and rotationally mounted between saidstator and said rotor to provide magnetic screening between said rotorand said stator; and control circuitry to control power supplied towindings on said rotor and said stator in dependence upon a desiredoutput rotational speed.
 2. An electric motor according to claim 1,wherein said stator and said rotor comprise a same number of windings.3. An electric motor according to claim 1, wherein said intermediatescreen comprises a plurality of inserts on at least one of said innerand outer surfaces of a highly electrically conductive material, saidplurality of inserts running at an offset angle to said longitudinalaxis to form a squirrel cage, said plurality of inserts on a samesurface being coupled at either end to form loops.
 4. An electric motoraccording to claim 3, wherein said intermediate screen comprises saidplurality of inserts on both said inner and outer surfaces.
 5. Anelectric motor according to claim 4, wherein said plurality of insertson said inner surface are arranged at different circumferentialpositions to said plurality of inserts on said outer surface.
 6. Anelectric motor according to claim 1, wherein said intermediate screencomprises a plurality of protrusions extending from said intermediatescreen on at least one of said inner and outer surfaces.
 7. An electricmotor according to claim 6, wherein said intermediate screen comprisessaid plurality of protrusions on both said inner and outer surfaces. 8.An electric motor according to claim 7, wherein said inner and saidouter surface comprise a same number of said plurality of protrusions,said plurality of protrusions on said inner surface being offsetcompared to said plurality of protrusions on said outer surface, suchthat a protrusion on said inner surface is not formed at a samecircumferential position as a protrusion on said outer surface.
 9. Anelectric motor according to claim 8, wherein said protrusions are formedby said inner and outer surfaces being undulating surfaces, saidundulating surfaces undulating at a substantially same rate and samedirection such that a thickness of said intermediate screen issubstantially constant around the circumference.
 10. An electric motoraccording to claim 1, wherein said rotor is mounted on an axle and saidintermediate screen is mounted on said axle.
 11. An electric motoraccording to claim 1, wherein said control circuitry comprises a statorcontrol unit and a rotor control unit, said rotor control unit beingmounted on said rotor.
 12. An electric motor according to claim 11,wherein said stator control unit is configured to control power suppliedto said windings on said stator to thereby generate a rotational forceon said intermediate screen; and said rotor control unit is configuredto control power supplied to said windings on said rotor to therebygenerate a rotational force between said intermediate screen and saidrotor, said rotational force exerted on said rotor being dependent on arotational speed of said intermediate screen due to said rotationalforce generated by said stator and said rotational force generatedbetween said intermediate screen and said rotor by power supplied tosaid windings of said rotor.
 13. An electric motor according to claim12, wherein said rotational forces generated by said stator control unitand said rotor control unit are combined via said intermediate screen togenerate a cumulative rotational force on said rotor.
 14. An electricmotor according to claim 11, further comprising a rotational positionsensor configured to generate rotational position information indicativeof a relative orientation of said intermediate screen and said rotor andsaid intermediate screen and said stator, said rotor control unitmounted on said rotor being coupled to a power supply supplied to saidrotor and configured to generate a rotor power supply waveform from saidpower supply in dependence upon said rotational position information anda desired output frequency of rotation of said rotor relative to saidintermediate screen and to apply said rotor c power supply waveform tosaid rotor; and said stator control unit being coupled to a power supplysupplied to said stator and configured to generate a stator power supplywaveform from said power supply in dependence upon said rotationalposition information of said intermediate screen and said stator and adesired output speed of rotation and to apply said stator power supplywaveform to said stator.
 15. A method of operating an electric motorapparatus, said electric motor apparatus comprising: a stator and arotor rotationally mounted coaxially with said stator; an intermediatescreen comprising a magnetic material rotationally mounted between saidstator and said rotor to provide magnetic screening between said rotorand said stator, said method comprising: controlling an outputrotational speed of said motor by providing power to windings on eithersaid rotor or said stator, or on said rotor and said stator.
 16. Anelectric motor apparatus comprising: a stator means and a rotor meansrotationally mounted coaxially with said stator means; an intermediatescreening means for providing magnetic screening comprising a magneticmaterial between said rotor means and said stator means, saidintermediate screening means being rotationally mounted between saidstator means and said rotor means; and control means for controllingpower supplied to windings on said rotor means and said stator means independence upon a desired output rotational speed.